Andreas,
The current environment is an uncontrolled home lab. Great variation in
temperature and humidity, hence the primary design will use the vhd200 oil
filled hermetic package resistor arrays. I plan on evaluating for
stability and accuracy to see if I need to ovenize it to meet my voltage
standard needs. I plan on building one version type with LTZ1000s and
another with a LM399s for less demanding requirements. The LTZ1000 will
definitely use the vhd200 resistor arrays and the LM399 may be able to use
the LTC5400 resistor arrays (or LTC1043). The vhd200 is a leaded package
so should have minimal mechanical stress on the package (hopefully). If
necessary, and likely needed for the best stability, the circuit can be
ovenized.
Lots of testing to do.
Randy
On Sun, Jul 27, 2014 at 3:17 AM, Andreas Jahn andreas_-_j...@t-online.de
wrote:
Hello Randy,
until now you have not written about your design accuracy needs and about
your environment conditions.
(humidity + temperature controlled lab or industrial environment or simply
your uncontrolled lab at home).
In your measurements you should also regard humidity as a significant
factor
especially with resistors in plastic package like the LTC5400.
Also the epoxy pcb is creating mechanical stress due to humidity to the
resistors
which can change the resistor value if not proper mechanical decoupled.
(Difficult to mandage with SMD parts).
I know that humidity is not easy to measure. And time constants are more in
the range of 4-7 days which makes it even more time consuming.
My personal opinion is that a LTC1043 based desing will perform better than
a desing with SMD metal film resistors like the LTC5400. And a shoot out
between
vhd200 and LTC1043 would also be interesting depending on temperature
range.
By the way. The term TC tracking which is used by the resistor
manufacturers may
be not the same that you expect in a lab condition.
Usually T.C. of a resistor is measured at 3 cardinal points -55 deg / 25
deg / 125 deg.
T.C. is then calculated out of these 3 values.
You probably want to have the dR / dT tracking around 25 degrees which is
not specified with this method.
So if you specifiy T.C. tracking you should do this with your temperature
range.
I hope that we will get feedback on your results some day.
With best regards
Andreas
Am 26.07.2014 17:56, schrieb Randy Evans:
I would like to thank all those you supplied ideas for matching resistors.
I have decided to test three approaches for now, the first is using
Vishay
vhd200 hermetically sealed foil resistors (three each at around $26 each),
the second is using LTC5400 resistor arrays, the third is a hybrid
approach
using a Vishay vhd200 for the most critical resistor pair and LTC5400
resistor arrays for the other two, and the fourth will likely be a LTC1043
switch capacitor doubler plus LTC5400s, although the latter is a lower
priority.
I plan on characterizing them over time and temperature to see the effects
on output stability for the best approach considering cost, complexity,
and
accuracy. Should be interesting if it works. It's been taking longer
than
I wanted or hoped but there are only so many hours in the day and my day
job is interfering with my play time.
Thanks for the help,
Randy
On Fri, Jul 25, 2014 at 1:04 PM, Ivan.Cousins ijcous...@frontier.com
wrote:
Randy,
You might want to look at:
Digikey PN 749-1052-1-ND (qty 1, $0.89)
or
Digikey PN Y4485-5K/5KBCT-ND (qty 1, $22.93)
Both parts are from Vishay.
My advice is to build something and then measure that something.
You are the best judge of your immediate design problem and measurement
capabilities.
I have found, after many years, that waiting for an Expert to hand you
your answer does not work.
No other Expert is as familiar with your present problem as you are.
After a number of build-measure cycles, you may become the new Expert.
If one waits for the ultimate answer then you may wait forever.
The art of engineering is to get close enough.
One can remain in the thinking stage forever, it can become an endless
loop.
Once you start the project by building and then measuring, you will be on
your way to an answer, your answer.
I am reminded of a saying everything is a transducer used here before.
The interactions in this case are the usual temperature, stress-strain,
humidity, resistor metal migration, etc.
Notice that I did not include time, (long term drift), on purpose.
It could be explained by one of the above or other, measurable quantity
over a measurement interval.
This is meant in the best spirit possible.
Best of luck in your project.
John C.
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